THEORISTS of the American labor movement long have argued over the causes of fluctuations in union membership and the prospects for future union growth. For a number of years, the hypothesis of Commons (1932) and Perlman (1966), which related union expansion to the business cycle, was the most widely accepted explanation of the fluctuations in union membership. the years, a number of critics such as Dunlop (1948), Shister (1953), and Bernstein (1954) challenged the business cycle hypothesis on the grounds that American unionism is too complex and diffuse a social phenomenon to be understood in such simple terms. They contended that a multicausal system (including the cycle) is necessary to account for the rise of trade unionism. Although union theorists have frequently provided statistical data that are consistent with their inferences concerning the major factors influencing union growth, prior to the study by Ashenfelter and Pencavel (1969) their hypotheses had not been tested on an empirical plane which could disprove the suggested causal relationship between such factors and union growth. Ashenfelter and Pencavel (A-P) used multiple regression analysis to estimate a single behavioral relationship, including social and political as well as economic variables, capable of explaining the growth of American trade unions membership in the period 1900-1960. Although the model of A-P apparently has identified the determinants of union growth, there is some question as to whether the model has equally identified the determinants of future union growth. In econometric time-series analysis, it is assumed that differential periods of time are homogeneous, except for differences in the explicit variables of the system that are measured, and for differences in random effects. Over long sweeps of time, this assumption may become very tenuous.' This issue lies at the heart of the recent debate among the so-called saturationists and the school concerning the future prospects of the rate of growth of the American labor movement.2 The saturationists argue that significant changes have occurred within the structure and composition of the American labor force which have caused the past determinants of union growth to be inoperable in the future.3 Proponents of the historical approach challenge the general validity of the influence of structural factors on the comparative propensity of workers to join unions. Taft (1963) contends that the saturationists argument assumes propensities and psychological attitudes which have not been proven. In fact, actual experience has shown these assumptions to be baseless, and not a scintilla of evidence has been presented to justify these conclusions. They maintain the labor movement increases its size in two ways at a modest pace over long spans of time and in sharp spurts at infrequent intervals, ... . and that the slow growth of unionism in the post World War II period easily can be fit into the theory.4 The debate between the saturationists and the school has not yet been resolved. The level of actual union membership has increased by 4,300,000 or 25.4% for the period 1953-1970, but the level of real union membership, as measured by the per cent of nonagricultural employment organized, has declined from 34.1 to 30.1. The recent success of the labor movement in organizing some difficult structural groups such as government employ-